1. Field of the Invention
The invention relates generally to photonic waveguide structures with improved electrical contacts for reducing lossy performance during optical modulation in high-speed optical modulators. More specifically, the invention relates to a photonic waveguide structure, where the provision of a conductive electrical contact isolates a propagating optical mode between the waveguide core and the electrodic contact while maintaining excellent electrical connectivity and low optical loss.
2. Background Art
The tunable non-linear optical properties of Silicon (Si) have been a major factor in the recent emergence of Si as a photonics material. With high performance computing (HPC) entering into the multi-core Central Processing Unit (CPU) era and linear scaling of computing bandwidth with the number of cores, communication bandwidth per core on multi-chips is becoming a performance bottleneck. Internal chip communication in Si chips using optical waveguides goes a long way in solving the aforementioned bottleneck that is pronounced in chip communication involving metallic interconnects. Thus, using Si as a light guiding element is the key to ultrafast HPC. Along with solving the problem of bandwidth requirement, Si photonics also provides for small circuit delay times, i.e., low latency, and for low power consumption. These, coupled with the relative ease of fabricating Si photonic devices using the typical CMOS processes with high yield and at low cost, enable the exploitation of the aforementioned advantages for realizing ultrafast circuits. It is possible to build a point-to-point communication network among a large number of CPU cores using Si photonic links, and maintain the scaled high performance with affordable manufacturing and energy costs.
In order to convert high-speed electrical data signals into optical form, an optical modulator is required. The basic requirements demanded of the optical modulator are high speed (>10 Gb/s), low power consumption, high ON/OFF extinction ratio, and compactness of size. To date, most of the reported Si high-speed modulators are based on the free-carrier plasma dispersion effect, or, the reduction of the refractive index of Si material with increased free carrier densities, i.e., electrons and holes. To utilize this effect for data modulation, the carrier densities in an optical waveguide have to be modulated in order to modulate the optical refractive index, and, thereby, the optical phase of the propagating laser light. The phase modulation can then be converted into optical intensity modulation (i.e. ON/OFF switching) by building the phase modulation waveguide into a Mach-Zehnder interferometer (MZI) or a ring resonator.
FIG. 1 shows a waveguide phase shifter 100, where the phase modulation waveguide is based on a modified MOS capacitor utilizing poly-Si instead of metal. The waveguide phase shifter 100 includes a thin gate oxide layer 125 (shown with boxed emphasis in FIG. 1) on top of the n-type Si slab 115 of the Si-on-oxide (SOI) substrate formed with an oxide (SiO2) layer 110 over an Si layer 105, and a highly conductive p-doped poly-Si waveguide rib 120 on top of the gate oxide layer 125. The poly-Si waveguide rib 120 forms a modified MOS capacitor embedded in an optical waveguide with the thin gate oxide layer 125 and the Si slab 115. The poly-Si waveguide rib 120 is also surrounded by the oxide layer 125.
Due to the large difference in optical refractive indices between Si/poly-Si and the surrounding oxide layer 125, the optical mode is confined around the thin gate oxide layer 125, with the surrounding oxide layer 125 serving as a cladding. Although provision of electrical contacts on top of the Si optical waveguide is preferred for efficient application of the electric field for high-speed optical modulation, low optical loss, and high extinction ratio, the abovementioned enormous optical absorption coefficient of metal contacts, along with the signal overlap of the propagating optical mode in the waveguide with the top surfaces of the poly-Si rib 120 and the Si slab 115, render the waveguide phase shifter 100 lossy. In order to mitigate this problem, metallic contacts are deployed on top of the poly Si layer at lateral extensions thereof.
FIG. 1 shows the n-contacts 130 and the p-contacts 135 made on the Si-slab 115 and the poly-Si rib 120 respectively. To provide for the aforementioned contacts, the Si-slab 115 and the poly-Si rib 120 have a high surface doping density. The surrounding oxide layer 125, to an extent, prevents the electric fields from spreading to the metallic contacts because of the confinement of the optical mode. By providing a relatively higher permittivity than that of free space, the thin gate oxide layer 125 provides for strong incident radiation polarization dependence of the optical mode.
When a voltage VD is applied between the p-type poly-Si rib 120 and the n-type Si-slab 115, a thin-charge layer is accumulated at both sides of the gate oxide, thereby modulating the carrier densities. The induced charge density change ΔNe (for electrons) and ΔNh (for holes) are related to the applied voltage as:
                                          Δ            ⁢                                                  ⁢                          N              e                                =                                    Δ              ⁢                                                          ⁢                              N                h                                      =                                          ɛ                                                      et                    ol                                    ⁢                                      t                    eff                                                              ⁢                              (                                                      V                    D                                    -                                      V                    FB                                                  )                                                    ,                            (        1        )            
where ε=ε0εr is the permittivity of the thin oxide layer 125, εo being the permittivity of free space and εr being the low-frequency dielectric constant of the thin oxide layer 125, e is the electronic charge, tol is the thickness of the oxide layer, teff is the effective charge layer thickness, and VFB is the flat band voltage of the MOS structure. The flat band voltage VFB is calculated by taking into account the charge present at the oxide-semiconductor interface and the thin oxide layer 125.
The refractive index changes caused by the accumulation of charge carriers may be obtained from Kramers-Kronig analysis that relates real (refractive index) and imaginary (absorption) parts of the dielectric function of a material. The change in refractive index Δneff is related to the phase shift Δφ in the optical mode as:
                              Δϕ          =                                                    2                ⁢                π                            λ                        ⁢            Δ            ⁢                                                  ⁢                          n              eff                        ⁢            L                          ,                            (        2        )            
where λ is the wavelength of radiation in free space, Δneff is the effective refractive index change in the waveguide, which is the difference between the effective refractive indices of the waveguide phase shifter 100 before and after accumulation of charges, and L is the active length of the phase shifter.
FIG. 2 shows an asymmetric MZI 200, including two identical capacitive MOS phase shifters 220 along two arms 210 of the MZI 200, for conversion of the phase modulation into an intensity modulation. The arms 210 are also made of Si. The asymmetric MZI 200 provides for an optical path difference between the two arms 210 so that a Y-fork 225 may be exploited to split and combine optical beams thereof. Input coupling is through optical input portion 205, and output coupling is through optical output portion 215.
FIG. 3 shows a ring resonator 300 coupled to a linear waveguide 302 for conversion of the phase modulation into an intensity modulation. When radiation at appropriate wavelengths is coupled to the ring resonator 310 (which includes phase modulation section 312 constructed from waveguide phase shifters) through the optical input portion 305 of the linear waveguide 302, the radiation undergoes intensity modulation over multiple optical paths traveled in the loop, which may be coupled to the optical output portion 315 of the linear waveguide 302.
Although the abovementioned waveguide phase shifter 100 largely avoids optical absorption by metal, the presence of many defects in the material lattice of poly-Si adds to the lossy nature of the structure, aided by a significant overlap of the optical mode with poly-Si. As a exemplary solution, FIG. 4 shows a waveguide phase shifter 400 with the waveguide rib 420 comprising a less conductive p-doped epitaxially grown Si (starting with epi-Si crystal instead of poly-Si). Thin layers of poly-Si 422 are deposited to make metallic p-contacts 435 with the top corners of the epi-Si waveguide rib 420. Like the waveguide phase shifter 100 in FIG. 1, the structure has a thin gate oxide layer 425 (shown with emphasis in FIG. 4) on top of the n-type Si slab 415 of the SOI substrate formed with an oxide layer 410 over an Si layer 405. The metallic n-contacts 430 are made on the Si-slab 415.
Although the improved waveguide phase shifter 400 of FIG. 4 significantly reduces optical loss due to the absorption of poly-Si, the structure may induce large resistance in modulators with sub-micron waveguide widths due to small contact area between the poly-Si layers 422 and the epi-Si waveguide rib 420, the typical width of the epi-Si waveguide rib 420 being 0.5 microns. As small waveguides are preferred for high modulation efficiency, the resistance becomes a performance deterrent factor. Therefore, the waveguide phase shifter 400 of FIG. 4 typically is suited for waveguide widths in excess of 3 microns. Additionally, the efficiency of both the waveguide phase shifters (100, 400) of FIGS. 1 and 4 is compromised due to the presence of lateral electrical contacts instead of contacts on top of the Si optical waveguide.
FIG. 5 shows a phase modulation waveguide 500 employing a lateral PN junction operated in depletion mode. The phase modulation waveguide 500 comprises an SOI substrate formed with an oxide layer 510 over an Si layer 505. The waveguide rib includes a p-doped Si 515 portion and an n-doped Si 520 portion forming a PN junction 540 along a lateral direction parallel to the surface of the oxide layer 510. Electrodes 530 and 535 are provided in contact with highly doped conductive regions of the p-doped Si 515 portion and the n-doped Si 520 portion. External electrical circuitry (not shown in FIG. 5) may be connected to the electrodes 530 and 535 to reverse bias the PN junction 540 such that the width of a carrier depletion zone is changed, thereby resulting in change of carrier densities in the optical mode. This results in a change of the refractive index along the phase modulation waveguide 500, thereby enabling modulation of light waves. The structure does not require electrical contacts on top of the waveguide rib (515, 520) and, therefore, the problem of lossy performance due to metal absorption is avoided.
An emerging type of high-speed modulators employs Ge/SiGe quantum wells epitaxially grown on an Si substrate through Low Pressure Chemical Vapor Deposition (LPCVD). FIG. 6 shows a quantum-well device 600 with an undoped Ge/SiGe quantum well layer 620 sandwiched by doped SiGe layers (a p-doped layer 615 and an n-doped layer 625). The quantum-well device 600 includes an Si substrate 605 over which doped SiGe layers (p-doped layers 610 and 615) are formed. The p-doped layer 610 is highly conductive in order to enable the provision of electrical p-contact 640. The sandwich structure is surrounded by an oxide layer 630, and n-contact 635 is provided on top of the n-doped layer 625. The n-contact 635 extends over the SiO2 layer 630 in a lateral direction thereof. When a voltage is applied across the doped SiGe layers (615, 625), the energy of the bound states inside the quantum well layer 620 reduces due to Coulombic interaction between electron-hole pairs. The effective band gap is modulated and there is a red shift in the interband absorption, leading to a decrease in the peak absorption value. This phenomenon is known as Quantum-Confined Stark Effect (QCSE), which is exploited to change optical absorption in the quantum well 620. Thus, modulation of laser light intensity passing through the quantum well 620 is possible without the need for an MZI or a ring resonator structure. The light may be guided by the oxide/SiGe and the SiGe/Si interfaces.
As efficient modulation requires embedding the Ge/SiGe quantum wells into a waveguide to provide for a long modulation length and small capacitance, there is a necessity to make electrical contact on top of the doped SiGe layers (610, 625). Typically, Au/Ti contacts are employed to make electrical contacts on top of the doped SiGe layers (610, 625). Again, this causes the structure to be lossy as there is significant overlap of the optical mode 650 with the metal.
Indium Tin Oxide (Tin-doped Indium Oxide, or, simply ITO) is a mixture of Indium Oxide (In2O3) and Tin Oxide (SnO2), typically 90% In2O3 and 10% SnO2 by weight. The high electrical conductivity and optical transparency of ITO aid utility in the field of optoelectronic devices. The transparency of ITO starts around an optical wavelength of 450 nm and may be extended into the near infrared region covering the wavelengths employed in optical interconnects. Depending on the ITO deposition techniques and composition, the electrical resistivity of ITO may measure in the 10-4 ohm-cm range.
ITO belongs to the family of transparent conductive oxides (TCOs), of which the most noted are Zinc Oxide doped with Al or Ga, and Titanium Oxide doped with Nb. Even though TCOs and other such materials are currently being investigated, ITO's superior properties have rendered it ahead of the pack. A variety of application and substrate dependent ITO thin film deposition techniques are available. However, a compromise between conductivity and transparency has to be reached during film deposition, as a high concentration of charge carriers will increase the conductivity of the material but decrease the transparency of the material. ITO thin films have been deposited by ion beam deposition, pulsed laser deposition, Chemical Vapor Deposition (CVD), and a variety of sputtering techniques such as Radio Frequency (RF) and Direct Current (DC) sputtering, co-sputtering, and reactive sputtering with oxygen and carbon dioxide additives to argon.
Due to the above-mentioned low optical absorption, coupled with high electrical conductivity, ITO has widely found use in optoelectronic devices such as transparent electrodes for liquid crystal displays (LCDs). LCD glass has transparent ITO coated electrical conductors used as electrodes plated onto each side thereof in contact with the liquid crystal fluid. The light propagates through liquid crystal cells with corresponding image information encoded thereof, and emerges out through an ITO coated screen. Typical transmission varies from 80% to 95%. in the normal incidence configuration. Several other applications utilizing ITO are possible, including solar cells, vertical cavity surface emitting lasers, and light emitting diodes (LEDs). Here, the optical propagation is also arranged surface normal to the ITO coated electrode mirror.
In addition to ITO, novel materials such as metamaterials comprising arrays of carbon nanotubes (CNTs) are being researched and being touted as building blocks of electronics and optoelectronics in a variety of applications. The aforementioned CNTs may be single-walled or multi-walled. CNT thin films possess unique electronic and optical properties that allow for utility in applications where transparency and conductivity are of prime importance. A transmission of 85% has been reported in CNT films, thereby lending credence to the competitiveness of CNTs with ITO. CNT films, having already found place in LEDs, displays and photovoltaics, also have shown promise as transparent microwave or EMI shielding coatings. With tunable dielectric properties, CNTs and engineered CNT metamaterials may displace ITO in the current applications to pave the way for the advent of novel components and systems once mature manufacturing processes are devised.